1. Field of the Invention
The present invention relates to a modulated light source, in particular for use as part of an opto-electronic communication network.
2. Description of the Related Art
In an opto-electronic communications network, it can be desirable to produce a high frequency modulated light source. One way of achieving this is to pass light of initially constant intensity through a modulator. Normally the modulator is formed from a modulator material whose optical properties depend on the electric field applied across it, so that modulating the electric field across the modulator material results in a modulation in the intensity of light passing through it. Examples of modulators used to modulate light from a laser outside the lasing medium include electro-absorption modulators, Mach-Zender interferometer modulators, and Fabry-Perot modulators.
The modulator material is normally biased with a quiescent voltage in order to bring it into an operating mode where an absorption edge of the modulator material is close to the wavelength of the light being modulated. A modulation voltage is superposed to the quiescent voltage in order to modulate the intensity of light passing through the modulator material, the quiescent voltage normally being constant in time or slowly varying as compared to the modulation voltage (typically, the quiescent voltage will vary with a frequency of less than 10 kHz, whereas the frequency of the modulation voltage will normally be between about 1 GHz and 100 GHz).
However, the absorption edge and other characteristics of the modulator material can vary if the temperature of the modulator changes, with the result that the modulator is no longer as effective. It is known therefore to mount the modulator on a temperature control element in order to keep the temperature of the modulator stable but this can be expensive.
It is an object of the present invention to address the above issues.
According to a first aspect of the present invention, there is provided a modulated light source comprising a laser and a modulator assembly for modulating the intensity of light produced by the laser, wherein the modulator assembly has: a modulator element with a modulating medium for modulating the intensity of light passing therethrough; electrodes for applying an electric field across the modulating medium; a temperature sensor for sensing the temperature of the modulating medium and for producing a temperature signal indicative of the temperature thereof; and, an electronic compensation circuit having an input for receiving the temperature signal from the temperature sensor, an output connected to the electrodes of the modulating medium for applying a quiescent voltage thereto, and a control unit for controlling the quiescent voltage according to a predetermined functional relationship with the received temperature signal.
The predetermined functional relationship between the temperature signal and the quiescent voltage can be chosen to suit the modulating medium employed, so that that if the modulating medium and/or the laser changes temperature during use, the applied quiescent voltage changes also, in such a way as to keep the modulator operating effectively. This reduces the need for a cooling element to keep the temperature of the modulator and/or the laser constant.
The control unit will preferably have a memory with a look up table, the look up table having data representative of a plurality of temperature signal values and data representative of a plurality of quiescent voltage values, wherein each temperature signal value is associated with a quiescent voltage value, and wherein the quiescent voltage value for each temperature signal value is chosen according to the predetermined relationship. This will allow the control unit to easily produce the correct quiescent voltage for each received temperature signal.
The control unit may also have a processor with a feed back algorithm programmed therein, such that if the quiescent voltage applied to the modulating medium is not equal to a target quiescent voltage, a change in the quiescent voltage is made, the change being a function of the difference between the applied quiescent voltage and the target quiescent voltage.
If the predetermined relationship between the quiescent voltage and the temperature signal is sufficiently simple, the control unit may comprise a transistor circuit having an output for outputting an output signal, an input for inputting an input signal and one or more transistors, the transistors being connected such that relationship between the input signal and the output signal is representative if the predetermined relationship between the quiescent voltage and the temperature signal. Such a transistor circuit will be particularly useful since it will reduce the need for a look up table if the predetermined relationship between the quiescent voltage and the temperature signal is linear or quasi linear.
The modulating medium will preferably have an absorption edge, the wavelength at which the absorption edge occurs being dependent on the electric field applied across the modulating medium. The quiescent voltage can then be set such that the wavelength at which the absorption edge occurs is close to the wavelength of the light produced by the laser (typically between about 1300 nm and 1550 nm), with the result that a small modulating voltage superposed on the quiescent voltage will cause a significant modulation in the amount of light that is able to pass through the modulator element.
If the wavelength of light produced by the laser remains constant, the predetermined functional relationship between the temperature signal and the quiescent voltage can be chosen to keep the absorption edge at the same wavelength when the temperature of the modulator changes.
However, in many situations the laser will self heat during use or its temperature will change as a consequence of ambient temperature variations, with the result that the wavelength it produces will change. Therefore the modulator element will preferably be in thermal contact with the laser so that the temperature of the laser can be inferred from the temperature of the modulator element. The predetermined functional relationship between the quiescent voltage and the temperature signal will then be chosen such that the absorption edge of the modulating medium follows the changes in the wavelength of light produced by the laser.
The modulating medium will preferably be formed from a semiconductor material. The semiconductor material may be a bulk material, in which case the absorption edge will be the band edge of the bulk material, the wavelength of the band edge being electric field dependent according to the Franz-Keldysch effect.
The modulating medium may be formed from a plurality of layers, such as a multiple quantum well structure, and the field dependence of the absorption edge may be due to the Quantum Confined Stark Effect. The laser may also be formed from a plurality of different semiconductor layers.
The different layers of semiconductor in the modulating medium and/or the laser will preferably be formed from combinations of In, Ga, As, Al, Sb or P in different proportions.
In order to facilitate fabrication, the laser and the modulator element may be formed on a common substrate as a monolithic device.
The modulator element will preferably have an input facet and an output facet through which light respectively enters and leaves the modulator element, and the laser will preferably have at least one light emitting facet through which light is emitted, the laser and the modulator element being arranged such that that the light emitting facet of the laser faces the input facet of the modulating element. This will make it easier for light produced by the laser to enter the modulator element.
The light emitting facet of the laser may be coincident or in contact with the input facet of the modulator element, such that the laser and the modulator are butt coupled Alternatively, a waveguide and/or a lens may be provided between the laser and the modulator element in order to reduce the amount of light that is lost between the laser and the modulator element. A mode converting element may be provided between the laser and the modulator to increase the amount of light from the laser entering the modulator.
An optical isolating element may be provided at the output of the modulator to reduce the amount of light reflected back into the modulator. Alternatively, the optical isolating element may be provided between the modulator and the laser.
The laser may be a distributed Bragg reflector laser or a distributed feedback laser. To stabilise the wavelength of emitted light, the laser may have an external Bragg grating. The external Bragg grating may be an formed from an optic fibre or a waveguide. The waveguide may be fabricated substantially from any one or more of the following: a polymer material, silicon, silicon nitride, or silicon oxide.
The laser and the modulator element will preferably be placed on a common chip carrier, so as to form a single packaged unit. The packaged unit may be fabricated using conventional techniques such as soldering, adhesive bonding, laser welding and thermo-compression bonding.
The temperature sensor may be placed on the chip carrier, in thermal contact with both the laser and the modulator element. Alternatively, the temperature sensor may be formed integrally with the laser or the modulator element. If the laser and the modulator element are formed as a monolithic device, the temperature sensor may be formed integrally therewith, in order to reduced the temperature gradient between the monolithic device and the sensor.
To provide a more accurate temperature reading or if the laser and the modulator are thermally isolated, the modulated light source may comprise two temperature sensors, one sensor for sensing the temperature of the laser, and the other sensor for sensing the temperature of the modulator element.
The or each sensor may be a thin film sensor such as a platinum resistance thermometer, or a semiconductor sensor, the semiconductor sensor being for example a p-n junction or a thermistor.
In one embodiment, the laser will be placed on a first chip carrier and the modulator element will be placed on a second chip carrier, a light guide or optical fibre being provided between the first and second chip carrier to feed light from the laser to the input facet of the modulator element. This will allow the modulator assembly to be used with different lasers, allowing more flexibility. A laser temperature sensor producing a laser temperature signal will preferably be provided on the first chip carrier to measure the temperature of the laser.
The first and second chip carriers may be placed adjacent to one another, or on different positions on a circuit board. Alternatively, the first and second chip carrier may be separated by a distance of several meters or several kilometers if an optic fibre is used to couple the first and second chip carriers.
A modulating circuit associated with the laser may be provided for modulating the laser light output with a digital code representative of the laser temperature. A detector connected to the compensation circuit may be provided with the modulator assembly to detect the light from the laser and read the digital code representative of the laser temperature, so that the compensation circuit can take into account the laser temperature when setting the quiescent voltage on the modulator element.
According to a second aspect of the present invention, there is provided a light modulator assembly for modulating light intensity comprising: a modulating medium for modulating the intensity of light passing therethrough; electrodes for applying an electric field across the modulating medium; a temperature sensor for sensing the temperature of the modulating medium and for producing a temperature signal indicative of the temperature thereof; and, an electronic compensation circuit having an input for receiving the temperature signal from the temperature sensor, an output connected to the electrodes of the modulating medium for applying a quiescent voltage thereto, and a control unit for controlling the quiescent voltage according to a predetermined functional relationship with the received temperature signal.